The specular infrared transmittance of mineral wool is reported and compared to the optical properties of the raw material, a silicate glass. A transmittance peak at 8.3 μm for the wool, not previously reported, is correlated to the reflectance minimum at the same wavelength for the bulk glass. It is concluded that the radiative behavior of this fibrous insulation cannot be described by gray models, neglecting the dispersion of the raw material. It is also demonstrated that the transmittance peak values are much higher than could be expected from the absorption of the glass. This is tentatively explained as an effect of forward scattering through very small angles, thus increasing the specular transmittance.